Gas treatment method

Abstract

Provided is a method capable of efficiently decomposing N2O contained in a gas to be treated which contains CO2 and N2O.　This method comprises: a step (a) in which an adsorption system including first adsorption towers and a second adsorption tower, which accommodate adsorbents capable of adsorbing CO2 and N2O, is prepared; a step (b) in which the gas to be treated which contains CO2 and N2O is passed through the first adsorption towers; a step (c) in which a first gas, which has passed through the adsorbent inside the first adsorption towers, is continuously sent to the second adsorption tower; a step (d) in which the N2O concentration of a second gas that is downstream along the passing direction from the adsorbent inside the second adsorption tower is measured; and a step (e) in which the step (c) is stopped at a given first timing when the N2O concentration of the second gas tends to increase. The first timing is a timing when at least the adsorbent accommodated in the first adsorption tower located most upstream has reached an absorption limit.

Description

Gas Treatment Method

[0001] The present invention relates to a gas treatment method, and more particularly to a method for treating a gas containing a mixture of carbon dioxide (CO2) and nitrous oxide (N2O).

[0002] Since the Industrial Revolution, the average temperature of the Earth has been rising, making global warming a pressing issue. Greenhouse gases known to cause global warming include CO2, methane (CH4), N2O, and chlorofluorocarbons. Of these gases, CO2 is the most abundant, followed by CH4, and then N2O.

[0003] On the other hand, when we look at the global warming potential (GWP), it has been reported that the GWP of CH4 is 25 times that of CO2, and that of N2O is 298 times that of CO2. For these reasons, the impact of N2O emissions on global warming cannot be ignored.

[0004] As a method for reducing NO emissions, for example, Patent Document 1 listed below discloses a technology in which exhaust gas containing NO emitted from an engine is irradiated with ultraviolet laser light from an excimer laser device to decompose the NO.

[0005] Japanese Patent Application Laid-Open No. 2021-088964

[0006] NO is emitted, for example, during the management of farm soil and livestock waste, the incineration of waste, and the treatment of wastewater. In particular, because NO is generated by microorganisms, the composition of the emitted gas is mostly atmospheric components, and NO is often present in low concentrations of around several hundred ppm.

[0007] In consideration of irradiating a gas to be treated with ultraviolet light to decompose NO, it is expected that the efficiency of decomposing NO will be extremely low if the concentration of NO in the gas to be treated is low. That is, as described in Patent Document 1, when irradiating the gas to be treated with excimer laser light, the irradiation dose of the excimer laser light must be extremely high. This means that the amount of energy required to decompose the NO contained in the gas to be treated, in other words, the amount of power input to the excimer laser light source, increases, which raises concerns about an increase in CO2 emissions due to increased power consumption.

[0008] In other words, even though the treatment is carried out with the aim of decomposing the NO contained in the gas to be treated from the viewpoint of curbing global warming, the treatment may induce an increase in emissions of CO, a greenhouse gas. Therefore, for gas to be treated that contains a low concentration of NO, the method of irradiating the gas with ultraviolet light with the aim of decomposing NO from the viewpoint of preventing global warming cannot be said to be effective.

[0009] One method for efficiently decomposing N2O contained in a gas to be treated at low concentrations is to adsorb N2O from the gas to be treated to increase its concentration, and then decompose the N2O.

[0010] Considering the above-mentioned NO emission sources, the gas to be treated typically contains CO at a higher concentration than NO. One possible method for concentrating NO involves passing the gas to be treated through an adsorbent to temporarily adsorb NO onto the adsorbent, and then desorbing NO from the adsorbent with a certain amount of NO adsorbed. This method allows the concentration of NO contained in the desorbed gas to be higher than that of NO contained in the gas to be treated. Therefore, it appears that photodecomposition of NO by irradiating the desorbed gas with, for example, ultraviolet light may improve the efficiency of NO decomposition compared to direct irradiation of ultraviolet light on the gas to be treated.

[0011] However, as described above, when the gas to be treated is passed through an adsorbent to adsorb the NO contained in the gas, not only NO but also CO is adsorbed by the adsorbent. There is a limit to the amount that can be adsorbed by the adsorbent. As described above, the gas to be treated usually contains a higher concentration of CO than NO. The amount of NO and CO adsorbed by the adsorbent depends on the concentration ratio of NO to CO in the gas to be treated. Therefore, when the gas to be treated is passed through the adsorbent, it is expected that a large amount of CO will be adsorbed by the adsorbent, and it will not be possible to ensure a sufficient amount of NO adsorption.

[0012] In order to ensure the amount of NO adsorption under these circumstances, one possible method would be to prepare an adsorption tower equipped with a large adsorbent and introduce the gas to be treated into the adsorption tower. However, this would require an extremely large device, and the effect relative to the size of the device would be limited, making the introduction of this method unrealistic.

[0013] As described above, even if a method of passing the gas to be treated through an adsorbent is adopted, it is difficult to efficiently increase the concentration of NO contained in the gas to be treated, and as a result, the effect of improving the decomposition efficiency of NO can be said to be extremely limited.

[0014] In view of the above problems, an object of the present invention is to provide a method that enables efficient decomposition of N2O contained in a gas to be treated that contains CO2 and N2O.

[0015] The gas treatment method according to the present invention includes the steps of: (a) preparing an adsorption system including a first adsorption tower containing an adsorbent capable of adsorbing CO2 and N2O; and a second adsorption tower connected downstream of the first adsorption tower and containing an adsorbent capable of adsorbing CO2 and N2O; (b) passing a gas to be treated containing CO2 and N2O through the first adsorption tower; (c) continuously feeding the first gas that has passed through the adsorbent in the first adsorption tower into the second adsorption tower; (d) measuring the N2O concentration of a second gas located downstream of the adsorbent in the second adsorption tower in the flow direction; and (e) stopping the execution of the step (c) at a predetermined first timing when the N2O concentration of the second gas measured in the step (d) shows an increasing trend, wherein the first timing is a timing when the adsorbent contained in at least the first adsorption tower located furthest upstream reaches its adsorption limit.

[0016] The first adsorption tower contains an adsorbent capable of adsorbing CO2 and NO. Therefore, when a gas to be treated containing CO2 and NO is passed through the first adsorption tower (step (b)), the CO2 and NO contained in the gas to be treated are initially sufficiently adsorbed by the adsorbent. However, as step (b) continues, the adsorption performance of the adsorbent in the first adsorption tower gradually deteriorates. When the adsorption performance of the adsorbent deteriorates significantly, the concentrations of CO2 and NO contained in the gas (first gas) that has passed through the adsorbent will rise to approximately the same levels as before the gas was passed through the adsorbent.

[0017] In this specification, the CO2 concentration contained in the gas before passing through the adsorbent is referred to as D C1 The CO2 concentration contained in the gas after passing through the adsorbent is D C2 When the difference between the two is expressed as |D C1 -D C2 |) / D C1 If the value of is 10% or less, the adsorbent may be considered to have reached its adsorption limit. For the same reason, the adsorption limit can also be defined by the concentration of NO. That is, in this specification, the NO concentration contained in the gas before passing through the adsorbent is defined as D N1The NO concentration contained in the gas after passing through the adsorbent is D N2 When the difference between the two is expressed as |D N1 -D N2 |) / D N1 If the value of is less than or equal to 10%, the adsorbent may be considered to have reached its adsorption limit.

[0018] As a result of intensive research by the present inventors, it has been confirmed that when a gas to be treated containing CO2 and NO is continuously supplied to an adsorbent, the NO already adsorbed in the adsorbent begins to desorb from the adsorbent, an adsorbable area is secured in the adsorbent, and CO2 is newly adsorbed. After that, when a gas to be treated containing CO2 and NO is further continuously supplied to the adsorbent, there is no longer an area in the adsorbent where CO2 can be adsorbed, and CO2 passes through the adsorbent without being substantially adsorbed by the adsorbent.

[0019] In other words, when the adsorbent in the first adsorption tower has reached its adsorption limit and the gas to be treated flows through the first adsorption tower, the CO2 and NO contained in the gas to be treated are no longer substantially adsorbed by the adsorbent in the first adsorption tower and pass through the first adsorption tower as is.

[0020] However, at this point, the adsorbent in the first adsorption tower has adsorbed CO2 contained in the target gas that was supplied before the adsorbent reached its adsorption limit. In addition, as described above, N2O contained in the target gas that was supplied before the adsorbent reached its adsorption limit is adsorbed by the adsorbent, and then desorbed by CO2 contained in the target gas and flows to a subsequent stage.

[0021] Therefore, when the adsorbent in the first adsorption tower reaches its adsorption limit, the adsorbent in the first adsorption tower adsorbs the adsorbent in an amount that is capable of absorbing CO contained in the target gas previously supplied, while the N2O contained in the target gas supplied before the adsorbent in the first adsorption tower reaches its adsorption limit is desorbed from the adsorbent. Therefore, the amount of CO2 per unit volume contained in the gas (first gas) that has passed through the adsorbent in the first adsorption tower decreases compared to the amount of CO2 per unit volume contained in the target gas due to the amount of CO2 adsorbed by the adsorbent in the first adsorption tower. In contrast, the amount of N2O per unit volume contained in the gas (first gas) that has passed through the adsorbent in the first adsorption tower remains almost unchanged compared to the amount of N2O per unit volume contained in the target gas. As a result, the concentration of N2O in the first gas is relatively higher than the concentration of N2O in the target gas.

[0022] The first gas that has passed through the adsorbent in the first adsorption tower is sent to the second adsorption tower in the subsequent stage. For the reasons described above, as time passes after the supply of the gas to be treated begins, the first gas exhibits a state in which the NO concentration is increased compared to the gas to be treated. On the other hand, after the adsorbent housed in the first adsorption tower reaches its adsorption limit, the adsorbent housed in the first adsorption tower cannot adsorb the CO contained in the gas to be treated, and therefore the first gas also contains CO. In other words, after the adsorbent housed in the first adsorption tower reaches its adsorption limit, the first gas exhibits a state in which the NO concentration is increased compared to the gas to be treated and contains CO and NO.

[0023] Like the first adsorption tower, the second adsorption tower also contains an adsorbent capable of adsorbing CO2 and NO. Therefore, when the first gas containing CO2 and NO is passed through the second adsorption tower, the CO2 and NO contained in the gas to be treated are initially sufficiently adsorbed by the adsorbent. Then, as the supply of the first gas to the second adsorption tower continues, the adsorption performance of the adsorbent in the second adsorption tower gradually decreases.

[0024] The adsorbent in the second adsorption tower exhibits the same phenomenon as the adsorbent in the first adsorption tower. That is, when the adsorbent in the second adsorption tower approaches or reaches its adsorption limit, of the CO and NO adsorbed by the adsorbent, NO tends to desorb from the adsorbent first. When this desorption occurs, an adsorbable area is secured in the adsorbent, and new CO is adsorbed. That is, when the concentrations of CO and NO in the gas (second gas) that has passed through the adsorbent in the second adsorption tower are compared, the timing at which the NO concentration begins to increase arrives before the timing at which the CO concentration begins to increase.

[0025] Therefore, focusing on the second gas that has passed through the adsorbent of the second adsorption tower, by stopping the delivery of the first gas to the second adsorption tower after the timing at which the NO concentration starts to increase and before the timing at which the CO concentration increases to a certain level (step (e)), a state in which a large amount of NO is adsorbed onto the adsorbent in the second adsorption tower is achieved. In particular, as described above, a state in which the NO concentration of the first gas is higher than that of the gas to be treated is achieved, and therefore, by stopping the delivery of the first gas to the second adsorption tower at the timing at which the NO concentration of the second gas shows an increasing trend (first timing), a state in which a large amount of NO is adsorbed onto the adsorbent in the second adsorption tower is achieved.

[0026] Therefore, after the first timing, for example, by desorbing the substance adsorbed by the adsorbent in the second adsorption tower, it is possible to generate a gas that contains NO at an extremely high concentration compared to the gas to be treated as the gas discharged from the adsorbent side of the second adsorption tower by the desorption process.

[0027] That is, the gas treatment method may include, after the step (e), a step (j) of desorbing the adsorbed substance adsorbed by the adsorbent in the second adsorption tower.

[0028] Several methods can be used for this step (j).

[0029] For example, step (j) may include heating the adsorbent in the second adsorption tower. The adsorbent in the second adsorption tower may be heated outside the adsorption system using waste heat generated by combustion or an exothermic reaction for multi-purpose use. Examples of such waste heat include waste heat from an incinerator, a chemical plant, a cogeneration system, and geothermal energy.

[0030] As another example, the step (j) may include a step of depressurizing the adsorbent in the second adsorption tower. The depressurization and heating of the adsorbent may be carried out simultaneously.

[0031] By desorbing the adsorbed substance adsorbed by the adsorbent in the second adsorption tower, a second gas containing a high concentration of NO is discharged from the second adsorption tower. Therefore, by decomposing the NO contained in this second gas, the NO contained in the gas to be treated is decomposed with high efficiency, which can contribute to preventing global warming.

[0032] That is, the gas processing method may include a step (k) of decomposing N2O contained in the second gas obtained after the start of the step (j).

[0033] This allows NO, which has a high GWP, to be converted into N or O, which are not greenhouse gases. The step (k) can be performed by irradiating the second gas with ultraviolet light or by passing the second gas through a plasma generation region. That is, the step (k) may be performed by feeding the second gas obtained after the start of the step (j) into a decomposition space formed by at least one of an ultraviolet light irradiation region and a plasma generation region.

[0034] The adsorbent housed in the first adsorption tower and the adsorbent housed in the second adsorption tower are made of adsorbents capable of adsorbing CO2 and NO. One example of such an adsorbent is an adsorbent whose main component is zeolite. Among zeolites, those having a skeletal structure of MOR, HEU, LTA, CHA, Beta, or FAU are particularly preferred, and hydrophilic zeolites with a Si / Al ratio of 1 or more and 8 or less are even more preferred. Another example is an adsorbent whose main component is MOF (Metal Organic Frameworks), CMS (Carbon Molecular Sieve), or the like.

[0035] When carrying out step (d), a gas sensor capable of measuring the concentration of NO may be installed, for example, in a flow path connected to the outlet of the second adsorption tower or at a position downstream of the adsorbent in the second adsorption tower, and the measurement results of the sensor may be monitored over time to detect a time period during which the concentration of NO in the second gas shows an increasing trend.

[0036] In the gas treatment method according to the present invention, the source of the gas to be treated containing CO and NO may be any source. For example, the gas to be treated may be gas generated from farm soil, livestock waste management sites, waste incineration plants, wastewater treatment plants, etc.

[0037] The adsorption system may include a plurality of the first adsorption towers connected in series, and the step (c) may be a step of feeding the first gas that has passed through the plurality of first adsorption towers into the second adsorption tower.

[0038] By providing multiple first adsorption towers connected in series, when the adsorbent in each first adsorption tower reaches its adsorption limit, CO in the gas is predominantly adsorbed by the respective adsorbents, while NO in the gas that was once adsorbed by the adsorbents is desorbed by CO and sent to the subsequent stage. As a result, the concentration of NO in the second gas discharged from the first adsorption tower located furthest downstream is further increased, and the amount of NO adsorbed by the adsorbent in the second adsorption tower is also further increased.

[0039] The adsorption system may be provided with a bypass flow path that bypasses each of the multiple first adsorption towers and can be connected to a next-stage adsorption tower, which is the first adsorption tower or the second adsorption tower, located immediately after it, and a discharge flow path that can be connected to each of the multiple first adsorption towers while blocking flow to the next-stage adsorption tower.

[0040] In this case, the gas treatment method may include the steps of: (f) measuring the NO concentration of the first gas corresponding to each of the plurality of first adsorption towers; (g) connecting the upstream of a first adsorption tower to be regenerated, which is a first adsorption tower where it has been confirmed that the NO concentration of the first gas measured in step (f) is substantially the same as the concentration of the gas to be treated, with the bypass flow path corresponding to the first adsorption tower to be regenerated, and stopping the inflow of gas into the first adsorption tower to be regenerated; and (h) desorbing the adsorbed substance adsorbed to the adsorbent in the first adsorption tower to be regenerated, with the first adsorption tower to be regenerated and the exhaust flow path corresponding to the first adsorption tower to be regenerated connected.

[0041] Once an adsorbent reaches its adsorption limit, it becomes virtually unable to adsorb the NO contained in a gas that contains NO even if that gas is subsequently supplied. Specifically, when the NO concentration of the gas (first gas) that has passed through the adsorbent in the first adsorption tower is measured and the measured value is substantially the same as the NO concentration contained in the gas to be treated, it is determined that the adsorbent has lost its ability to adsorb NO. On the other hand, the adsorption performance of the adsorbent can be regenerated by desorbing the substances adsorbed on the adsorbent.

[0042] Hereinafter, one of the first adsorption towers will be referred to as the “first adsorption tower to be regenerated.” This first adsorption tower to be regenerated is a first adsorption tower in which the adsorbent stored therein no longer functions to adsorb NO, and in which the adsorption performance of the adsorbent stored therein is to be regenerated.

[0043] The adsorption system has a bypass flow path that bypasses each of the multiple first adsorption towers and can be connected to the adsorption tower (first adsorption tower or second adsorption tower) located immediately behind it. Once the first adsorption tower whose adsorption performance is to be regenerated (i.e., the first adsorption tower to be regenerated) is determined, the upstream and downstream sides of the first adsorption tower to be regenerated are connected through the bypass flow path that bypasses this "first adsorption tower to be regenerated," while the flow of gas through the first adsorption tower to be regenerated is stopped. For convenience, the adsorption tower (first adsorption tower or second adsorption tower) located next to the first adsorption tower to be regenerated is referred to as the "next-stage adsorption tower."

[0044] For example, if the first adsorption tower to be regenerated is the first adsorption tower located most upstream among the multiple first adsorption towers, the supply path for the gas to be treated and the first adsorption tower serving as the next adsorption tower, which is located immediately downstream of the first adsorption tower to be regenerated, are connected through a bypass flow path, and the supply of the gas to be treated to the first adsorption tower to be regenerated is stopped. Furthermore, if the first adsorption tower to be regenerated is the first adsorption tower located in an intermediate region among the multiple first adsorption towers, the first adsorption tower located immediately upstream and the first adsorption tower serving as the next adsorption tower, which is located immediately downstream of the first adsorption tower to be regenerated, are connected through a bypass flow path, and the supply of the first gas to the first adsorption tower to be regenerated is stopped. Furthermore, if the first adsorption tower to be regenerated is the first adsorption tower located most downstream among the multiple first adsorption towers, the first adsorption tower located immediately upstream and the second adsorption tower, which is located immediately downstream of the first adsorption tower to be regenerated, are connected through a bypass flow path, and the supply of the first gas to the first adsorption tower to be regenerated is stopped.

[0045] As a result, the supply of gas to the first adsorption tower to be regenerated is cut off, and therefore, with the first adsorption tower to be regenerated and the discharge flow path connected, the adsorbed substance adsorbed to the adsorbent in the first adsorption tower to be regenerated is desorbed, and gas containing the adsorbed substance is discharged through this discharge flow path. Because the discharge flow path is a flow path that can be connected to the first adsorption tower to be regenerated while flow to the next-stage adsorption tower is cut off, gas containing the adsorbed substance adsorbed to the adsorbent in the first adsorption tower to be regenerated does not flow into the next-stage adsorption tower.

[0046] Furthermore, since the upstream and downstream sides of the first adsorption tower to be regenerated are connected through a bypass flow path, it is possible to regenerate the adsorbent in the first adsorption tower to be regenerated while continuing to supply the gas to be treated.

[0047] As the step (h) of desorbing the adsorbed substance adsorbed by the adsorbent in the first adsorption tower to be regenerated, several methods can be adopted.

[0048] For example, step (h) may include heating the adsorbent in the first adsorption tower to be regenerated. This heating may be performed by utilizing waste heat generated by combustion or an exothermic reaction for multi-purpose use outside the adsorption system. Examples of such waste heat include waste heat from an incinerator, a chemical plant, a cogeneration system, and geothermal energy.

[0049] As another example, the step (h) may include a step of depressurizing the adsorbent in the first adsorption tower to be regenerated. The depressurization and heating of the adsorbent may be performed simultaneously.

[0050] The gas treatment method includes, after completion of step (h), step (i) of shielding the bypass flow path corresponding to the first adsorption tower to be regenerated to connect the upstream of the first adsorption tower to be regenerated with the first adsorption tower to be regenerated, and step (g), step (h), and step (i) may be performed by sequentially switching the first adsorption tower to be regenerated from among a plurality of first adsorption towers.

[0051] According to this method, when the adsorption system has multiple first adsorption towers connected in series, the adsorbents contained in each of the multiple first adsorption towers can be regenerated sequentially without stopping the supply of the gas to be treated.

[0052] According to the present invention, it is possible to efficiently decompose N2O contained in a gas to be treated that contains CO2 and N2O.

[0053] 1 is a block diagram schematically showing an example of a gas processing system capable of executing a first embodiment of a gas processing method according to the present invention. FIG. 2 is a flowchart schematically showing an execution procedure of an embodiment of the gas processing method according to the present invention. FIG. 3 is a graph showing changes over time in the NO concentration and CO concentration in a gas obtained after passing a gas to be processed containing NO and CO through an adsorbent. FIG. 4 is a cross-sectional view schematically showing an example of the configuration of a decomposition tower. FIG. 5 is a schematic side view of an excimer lamp constituting a decomposition device. FIG. 6 is a schematic cross-sectional view of the excimer lamp shown in FIG. 5 taken along line A1-A1. FIG. 7 is a schematic cross-sectional view of an excimer lamp having a different structure constituting a decomposition device. FIG. 8 is a schematic cross-sectional view of the excimer lamp shown in FIG. 8 taken along line A2-A2. FIG. 9 is a schematic side view of an atmospheric pressure plasma generation device constituting a decomposition device. FIG. 10 is a schematic cross-sectional view of the excimer lamp shown in FIG. 10 taken along line A3-A3. FIG. 11 is a block diagram schematically showing another example of a gas processing system. Fig. 1 is a block diagram schematically showing another example of a gas processing system. Fig. 2 is a block diagram schematically showing another example of a gas processing system. Fig. 3 is a block diagram schematically showing another example of a gas processing system. Fig. 4 is a block diagram schematically showing another example of a gas processing system. Fig. 5 is a block diagram schematically showing an example of a gas processing system capable of executing a second embodiment of a gas processing method according to the present invention. Fig. 6 is a flowchart schematically showing the execution procedure of the second embodiment of a gas processing method according to the present invention.

[0054] The present invention relates to a gas processing method for increasing the concentration of N2O in a gas to be processed containing CO2 and N2O, to a concentration higher than that of the gas to be processed. Hereinafter, an embodiment of this method will be described with reference to the drawings as appropriate.

[0055] It should be noted that the following drawings are schematic illustrations, and the dimensional ratios on the drawings do not necessarily match the actual dimensional ratios. Furthermore, the dimensional ratios between the drawings may not match each other.

[0056] First Embodiment A first embodiment of the gas processing method according to the present invention will be described.

[0057] 1 is a block diagram schematically illustrating an example of a system (hereinafter referred to as a "gas processing system 1") capable of implementing a first embodiment of a gas processing method according to the present invention. The gas processing system 1 shown in FIG. 1 includes a gas supply source 3, a first adsorption tower 10, a second adsorption tower 20, and a decomposition tower 30.

[0058] In addition, in the following description, when it is necessary to extract and explain the areas corresponding to the first adsorption tower 10 and the second adsorption tower 20 among the elements included in the gas processing system 1, these areas may be referred to as the "adsorption system 2."

[0059] The first adsorption tower 10 is disposed upstream of the second adsorption tower 20. In the adsorption system 2 within the gas processing system 1 shown in FIG. 1 , it is assumed that a plurality of first adsorption towers 10, 10, ... are connected in series with one another. Hereinafter, the first adsorption towers 10, 10, ... disposed upstream of the second adsorption tower 20 may be collectively referred to as a "first adsorption unit 18" as appropriate.

[0060] FIG. 1 illustrates a case in which the first adsorption unit 18 includes three first adsorption towers 10. These three first adsorption towers 10 are connected in series. Hereinafter, when it is necessary to distinguish between these three first adsorption towers 10, they may be designated by the suffixes A, B, and C, in order from the most upstream side. In this description using these suffixes, the first adsorption tower 10 located most upstream will be referred to as the "first adsorption tower 10A," the first adsorption tower 10 located downstream of the first adsorption tower 10A will be referred to as the "first adsorption tower 10B," and the first adsorption tower 10 located downstream of the first adsorption tower 10B will be referred to as the "first adsorption tower 10C." Furthermore, when it is not necessary to distinguish between the first adsorption towers 10A, 10B, and 10C, they will be collectively referred to simply as the "first adsorption towers 10."

[0061] 1 is merely an example, and when the first adsorption unit 18 includes a plurality of first adsorption towers 10, 10, ..., the number of first adsorption towers 10 is not limited to three. That is, when the first adsorption unit 18 includes a plurality of first adsorption towers 10, 10, ..., the number of first adsorption towers 10 may be two, or may be four or more.

[0062] Furthermore, in the gas processing system 1 (adsorption system 2), it is also possible that the first adsorption unit 18 includes a single first adsorption tower 10. This configuration will be described later with reference to FIG.

[0063] The gas supply source 3 is a supply source of the gas Gt to be treated that contains CO and NO. As an example, the gas supply source 3 is composed of cylinders, piping, etc. that collect gases generated from farm soil, livestock waste management areas, waste management areas, waste incineration plants, wastewater treatment plants, etc.

[0064] The first adsorption tower 10 accommodates an adsorbent 11. The adsorbent 11 is made of a material capable of adsorbing CO and N O, and includes, for example, zeolite, MOF, CMS, or the like as a main component.

[0065] The second adsorption tower 20 accommodates therein an adsorbent 21. The adsorbent 21 is made of a material capable of adsorbing CO and N O, and examples of the material include zeolite, MOF, and CMS as its main component.

[0066] The decomposition tower 30 houses therein a device (referred to as a "decomposition device") capable of generating a decomposition space for decomposing NO, and is disposed downstream of the second adsorption tower 20. The decomposition device will be described in detail later.

[0067] As described above, the gas processing system 1 (adsorption system 2) shown in FIG. 1 is provided with three first adsorption towers 10A, 10B, and 10C.

[0068] The first adsorption tower 10A, which is located most upstream, is connected to the gas supply source 3 via a pipe 40. The first adsorption tower 10B, which is located downstream of the first adsorption tower 10A, is connected to the first adsorption tower 10A via a pipe 41. The first adsorption tower 10C, which is located downstream of the first adsorption tower 10B, is connected to the first adsorption tower 10B via a pipe 42. In the gas treatment system 1 (adsorption system 2, first adsorption unit 18) of this embodiment, the first adsorption tower 10C is the first adsorption tower 10 located most downstream. The first adsorption tower 10C is connected to the second adsorption tower 20 via a pipe 43. The second adsorption tower 20 is connected to the decomposition tower 30 via a pipe 48.

[0069] 1 , in the gas processing system 1 of the present embodiment, a three-way valve 78 is provided in the piping 48. This three-way valve 78 is configured to be able to switch between sending the gas flowing through the piping 48 to the decomposition tower 30 or to the exhaust flow path 58 side.

[0070] 1, a gas sensor 60 is provided in the pipe 40 connecting the gas supply source 3 and the first adsorption tower 10A. This gas sensor 60 is configured to be able to measure the NO concentration of the gas to be treated Gt sent from the gas supply source 3 to the first adsorption tower 10. However, if the NO concentration of the gas to be treated Gt is known or if a gas sensor capable of measuring the NO concentration is provided on the gas supply source 3 device side, the gas sensor 60 is not necessary.

[0071] 1 , a gas sensor 68 is provided in the pipe 48 connecting the second adsorption tower 20 and the decomposition tower 30 located downstream of the second adsorption tower 20. The gas sensor 68 is configured to be able to measure the NO concentration of the second gas G2 flowing out from the second adsorption tower 20. The gas sensor 68 may be installed inside the second adsorption tower 20, downstream of the adsorbent 21 in the second adsorption tower 20.

[0072] Each gas sensor (60, 61, 62, 63, 68) may be capable of measuring the concentration of CO2 in addition to the concentration of N2O contained in the flowing gas.

[0073] 1 functions to decompose N2O contained in the second gas G2 that is sent out from the second adsorption tower 20 and flows in through the pipe 48. The detailed configuration of the decomposition tower 30 will be described later.

[0074] 1 is provided with a heating unit 90 for heating each of the first adsorption towers 10 and the second adsorption towers 20. More specifically, in this embodiment, the heating unit 90 functions as a heat exchanger that performs heat exchange between a high-temperature fluid E1 flowing through a pipe 91 and each of the first adsorption towers 10 and the second adsorption towers 20. The fluid E1 is a high-temperature gas or liquid supplied from outside the gas processing system 1. A more specific example is a gas or liquid whose temperature has been increased by exhaust heat from an incinerator, factory exhaust heat from a chemical plant or the like, exhaust heat from a cogeneration system, geothermal energy, or the like.

[0075] Each heating section 90 is provided for the purpose of desorbing substances adsorbed in the adsorbents 11, 21 provided in the corresponding first adsorption tower 10 and second adsorption tower 20. Note that, as a method for desorbing substances adsorbed in the adsorbents 11, 21, in addition to a method for heating each adsorbent 11, 21, a method for depressurizing each adsorbent 11, 21 can also be used, or both can be used in combination.

[0076] 2 is a flow chart showing a typical procedure for carrying out the gas processing method according to the present embodiment. In the following description, step numbers shown in the flow chart in FIG. 2 will be referred to as appropriate.

[0077] (Step #1) As illustrated in Figure 1, an adsorption system 2 is prepared, which includes a first adsorption tower 10 and a second adsorption tower 20. More specifically, a gas processing system 1 is prepared, which includes the adsorption system 2, a gas supply source 3, and a decomposition tower 30.

[0078] This step #1 corresponds to process (a).

[0079] (Step #2) The gas to be treated Gt is supplied to the first adsorption tower 10A from the gas supply source 3 via the pipe 40. As described above, the gas to be treated Gt is a gas containing CO2 and N2O.

[0080] An on-off valve (not shown) may be provided at the connection between the gas supply source 3 and the piping 40 or in the middle of the piping 40, and by controlling this on-off valve, the gas to be treated Gt from the gas supply source 3 may be supplied to the first adsorption tower 10A.

[0081] As described above, the first adsorption tower 10A contains the adsorbent 11 capable of adsorbing CO and NO. Therefore, when the gas to be treated Gt is supplied to the first adsorption tower 10A, the adsorbent 11 installed in the first adsorption tower 10A starts adsorbing the CO and NO contained in the gas to be treated Gt.

[0082] In the initial stage, the adsorbent 11 in the first adsorption tower 10A has high adsorption performance, so that both CO and NO contained in the gas Gt to be treated are adsorbed. As a result, the first gas G1 that has passed through the first adsorption tower 10A is a gas from which CO and NO have been substantially removed. Therefore, this first gas G1 can be discharged directly to the outside of the gas treatment system 1. Specifically, after passing through the first adsorption tower 10B, the first adsorption tower 10C, and the second adsorption tower 20, it can be discharged through the discharge flow path 58. In this case, the three-way valve 78 may be controlled to connect the piping 48 and the discharge flow path 58.

[0083] This step #2 corresponds to process (b).

[0084] (Step #3) As time passes after the supply of the gas to be treated Gt is started, the adsorption performance of the adsorbent 11 in the first adsorption tower 10A gradually begins to deteriorate. Specifically, at the initial point when execution of step #2 is started, NO contained in the gas to be treated Gt is adsorbed by the adsorbent 11 in the first adsorption tower 10A, so the NO concentration of the first gas G1 discharged from the first adsorption tower 10A is sufficiently low. However, as time passes and the adsorption performance of the adsorbent 11 in the first adsorption tower 10A deteriorates, the amount of NO contained in the gas to be treated Gt being supplied becomes greater than the amount of NO that can be adsorbed by the adsorbent 11 per unit time. As a result, the NO concentration of the first gas G1 that has passed through the adsorbent 11 begins to show an increasing trend.

[0085] 3 is a graph showing the time-dependent changes in the NO concentration and CO concentration of a first gas G1 that has passed through the first adsorption tower 10A, measured with a gas sensor 61. The graph shows the time-dependent changes in the NO concentration and CO concentration of the first gas G1, measured with a gas sensor 61. In FIG. 3, the left vertical axis represents the CO concentration of the first gas G1, the right vertical axis represents the NO concentration of the first gas G1, and the horizontal axis represents the elapsed time.

[0086] 3, the concentrations of NO and CO contained in the first gas G1 were both below the detection limit until approximately 2,340 seconds had elapsed since the target gas Gt began to be supplied to the first adsorption tower 10A. In other words, it is estimated that the CO and NO contained in the target gas Gt were almost completely adsorbed by the adsorbent 11 contained in the first adsorption tower 10A during this time.

[0087] Thereafter, as the target gas Gt continued to be supplied to the first adsorption tower 10A, the NO concentration in the first gas G1 began to increase, while the CO concentration in the first gas G1 remained below the detection limit.

[0088] The NO concentration contained in the first gas G1 showed an increasing trend until about 2,500 seconds had elapsed since the start of supplying the first gas G1 to the first adsorption tower 10A, and then showed a decreasing trend. At approximately the same time as the NO concentration contained in the first gas G1 started to show a decreasing trend, the CO concentration contained in the first gas G1 showed an increasing trend.

[0089] Even after the NO concentration in the first gas G1 starts to show an increasing trend, the gas to be treated Gt containing CO and NO continues to be supplied to the first adsorption tower 10A, and the adsorbent 11 in the first adsorption tower 10A eventually reaches its adsorption limit. When the adsorbent 11 in the first adsorption tower 10A reaches its adsorption limit, the NO concentration in the first gas G1 becomes equivalent to the NO concentration in the gas to be treated Gt.

[0090] 3, it is confirmed that the timing at which the NO concentration of the first gas G1 shows an increasing trend occurs before the timing at which the CO concentration of the first gas G1 shows an increasing trend. The reason for this is presumably that the NO adsorbed by the adsorbent 11 in the first adsorption tower 10A is pushed out and desorbed by the adsorption force of CO in the target gas Gt onto the adsorbent 11. In other words, when an adsorbable amount of NO has been adsorbed onto the adsorbent 11 in the first adsorption tower 10A, if the target gas Gt is further supplied to the adsorbent 11, the NO adsorbed by the adsorbent 11 is pushed out by the adsorption force of CO in the target gas Gt onto the adsorbent 11, and as a result, CO is newly adsorbed in the area where NO was previously adsorbed.

[0091] The above phenomenon continues for a certain period of time, but during this time period, the adsorbent 11 in the first adsorption tower 10A is nearly at its adsorption limit, so that the adsorbent 11 is unable to completely adsorb the CO in the gas to be treated Gt, and the CO begins to pass through the adsorbent 11. As a result, the concentration of CO in the first gas G1 also begins to show an increasing trend.

[0092] In the gas treatment system 1 shown in FIG. 1 , the first gas G1 delivered from the first adsorption tower 10A is subsequently fed to the first adsorption tower 10B through the pipe 41. As described above, the NO concentration in the first gas G1 increases over time, eventually reaching a value similar to that of the target gas Gt. The CO concentration contained in the first gas G1 delivered from the first adsorption tower 10A also increases over time and eventually reaches a predetermined value. In other words, the first gas G1 delivered from the first adsorption tower 10A to the first adsorption tower 10B can be said to be a gas containing NO and CO, similar to the target gas Gt. Therefore, CO and NO are adsorbed by the adsorbent 11 in the first adsorption tower 10B, similar to the first adsorption tower 10A.

[0093] 3, the adsorbent 11 in the first adsorption tower 10A initially adsorbs NO in the gas Gt to be treated, but the adsorbed NO is pushed out by CO in the gas Gt, and CO is adsorbed in this pushed-out region. That is, at this point in time, CO is predominantly adsorbed by the adsorbent 11 in the first adsorption tower 10A.

[0094] Therefore, the adsorbent 11 in the first adsorption tower 10A approaches or reaches its adsorption limit while a portion of the CO contained in the target gas Gt is adsorbed by the adsorbent 11 in the first adsorption tower 10A. That is, the first gas G1 discharged from the first adsorption tower 10A becomes a gas in which a portion of the CO contained in the target gas Gt is adsorbed by the adsorbent 11 in the first adsorption tower 10A. Therefore, the amount of CO contained per unit volume of the first gas G1 discharged from the first adsorption tower 10A is lower than the amount of CO contained per unit volume of the target gas Gt. Meanwhile, the adsorbent 11 in the first adsorption tower 10A contains almost no NO contained in the target gas Gt. As a result, the concentration of NO in the first gas G1 discharged from the first adsorption tower 10A is relatively higher than the concentration of NO in the target gas Gt.

[0095] In other words, the first gas G1 supplied to the first adsorption tower 10B located downstream of the first adsorption tower 10A is the same as the gas to be treated Gt supplied to the first adsorption tower 10A in that it contains CO2 and N2O, but the concentration of N2O is higher than that of the gas to be treated Gt.

[0096] Next, attention will be focused on the adsorbent 11 in the first adsorption tower 10B. The phenomenon that occurs when the first gas G1 containing CO and NO is supplied to the first adsorption tower 10B is similar to the phenomenon that occurs when the treatment target gas Gt containing CO and NO is supplied to the first adsorption tower 10A. Hereinafter, for convenience of explanation, when it is necessary to distinguish between the first gas G1 delivered from the first adsorption tower 10A, the first gas G1 delivered from the first adsorption tower 10B, and the first gas G1 delivered from the first adsorption tower 10C, they will be referred to as the "first gas G1A," the "first gas G1B," and the "first gas G1C," respectively.

[0097] Similar to the adsorbent 11 installed in the first adsorption tower 10A, in the initial stage, the adsorbent 11 installed in the first adsorption tower 10B begins to adsorb CO2 and NO contained in the first gas G1A. However, when the amount of NO that can be adsorbed by the adsorbent 11 installed in the first adsorption tower 10B eventually reaches its limit, the NO concentration of the first gas G1B that has passed through the first adsorption tower 10B begins to show an increasing tendency.

[0098] In the initial stage, the first adsorption tower 10B can adsorb NO in the first gas G1A sent from the first adsorption tower 10A, and therefore the first gas G1B that has passed through the first adsorption tower 10B can be directly discharged to the outside of the gas treatment system 1. Specifically, after passing through the first adsorption tower 10C and the second adsorption tower 20, the first gas G1B can be discharged through the discharge flow path 58.

[0099] As the process of feeding the first gas G1A from the first adsorption tower 10A to the first adsorption tower 10B continues, the concentration of NO contained in the first gas G1B discharged from the first adsorption tower 10B increases over time, and eventually reaches a value similar to that of the first gas G1A. The first gas G1B fed from the first adsorption tower 10B to the first adsorption tower 10C can be said to be a gas containing NO and CO, similar to the target gas Gt. Therefore, CO and NO are adsorbed by the adsorbent 11 in the first adsorption tower 10C, similar to the first adsorption tower 10B.

[0100] The adsorbent 11 in the first adsorption tower 10B approaches or reaches its adsorption limit in a state in which a portion of the CO contained in the first gas G1A is adsorbed by the adsorbent 11 in the first adsorption tower 10B. That is, the first gas G1B discharged from the first adsorption tower 10B is a gas in which a portion of the CO contained in the first gas G1A delivered from the first adsorption tower 10A to the first adsorption tower 10B is adsorbed by the adsorbent 11 in the first adsorption tower 10B. Therefore, for the same reason as above, the concentration of NO in the first gas G1B delivered from the first adsorption tower 10B becomes relatively higher than the concentration of NO in the first gas G1A.

[0101] 1 , the first gas G1B discharged from the first adsorption tower 10B is continuously fed to the subsequent first adsorption tower 10C. That is, as with the first adsorption towers 10A and 10B, as gas supply continues, the adsorbent 11 in the first adsorption tower 10C eventually approaches or reaches its adsorption limit. Therefore, the NO concentration in the first gas G1C delivered from the first adsorption tower 10C becomes relatively higher than the NO concentration in the first gas G1B.

[0102] 1 , the first adsorption tower 10C corresponds to the first adsorption tower 10 located most downstream. In this manner, in the gas processing system 1 of the present embodiment, the first gas G1 (G1C) discharged from the most downstream first adsorption tower 10 (10C) passes through the multiple first adsorption towers 10, 10, ... connected in series, so that the NO concentration of the first gas G1 (G1C) is significantly increased compared to the gas to be treated Gt.

[0103] (Step #4) In the gas processing system 1 shown in Figure 1, the second adsorption tower 20 is disposed downstream of the first adsorption tower 10 (10C), which is located at the most downstream side of the first adsorption unit 18. The first gas G1C discharged from the first adsorption tower 10C is then fed into the second adsorption tower 20 through the pipe 43.

[0104] This step #4 corresponds to process (c).

[0105] (Step #5) As described above, the adsorbent 11 in the first adsorption tower 10C also reaches a state in which CO is predominantly adsorbed over time, similar to the adsorbent 11 in the first adsorption tower 10A. Therefore, the concentration of NO in the first gas G1C sent from the first adsorption tower 10C to the second adsorption tower 20 becomes even higher than the concentration of NO in the first gas G1B sent from the first adsorption tower 10B to the first adsorption tower 10C.

[0106] To summarize the above, if the NO concentration of the gas to be treated Gt supplied from the gas supply source 3 to the first adsorption tower 10A is Dt, the NO concentration of the first gas G1A sent from the first adsorption tower 10A to the first adsorption tower 10B is D1A, the NO concentration of the first gas G1B sent from the first adsorption tower 10B to the first adsorption tower 10C is D1B, and the NO concentration of the first gas G1C sent from the first adsorption tower 10C to the second adsorption tower 20 is D1C, it can be understood that the respective relationships are Dt < D1A < D1B < D1C.

[0107] In other words, the first gas G1C supplied to the second adsorption tower 20 is the same as the gas to be treated Gt in that it contains CO2 and N2O, but the concentration of N2O is significantly higher than that of the gas to be treated Gt.

[0108] The second adsorption tower 20 contains an adsorbent 21. As described above, the adsorbent 21 is made of a material capable of adsorbing CO and NO, similar to the adsorbent 11 in the first adsorption tower 10. Therefore, when the first gas G1C delivered from the first adsorption tower 10C is supplied to the second adsorption tower 20, a phenomenon similar to that of the adsorbent 11 in the first adsorption tower 10 occurs. That is, in the initial stage, the adsorbent 21 installed in the second adsorption tower 20 begins to adsorb CO and NO contained in the first gas G1C. When the amount of NO that can be adsorbed by the adsorbent 21 installed in the second adsorption tower 20 eventually reaches its limit, the NO concentration of the second gas G2 discharged after passing through the second adsorption tower 20 begins to show an increasing trend.

[0109] In step #5, the NO concentration of the second gas G2 discharged from the second adsorption tower 20 is monitored. As a specific example, the NO concentration of the second gas G2 is measured by a gas sensor 68 provided in the pipe 48 through which the second gas G2 flows.

[0110] This step #5 corresponds to process (d).

[0111] (Steps #6 and #7) In the initial stage when the supply of the first gas G1C from the first adsorption tower 10C to the second adsorption tower 20 begins, NO in the first gas G1C can be adsorbed by the adsorbent 21 in the second adsorption tower 20. During this time period, the NO concentration in the second gas G2 that has passed through the second adsorption tower 20 is sufficiently low, so the second gas G2 can be directly discharged to the outside of the gas treatment system 1. In the configuration of the gas treatment system 1 shown in FIG. 1 , the second gas G2 discharged from the second adsorption tower 20 can be discharged through the discharge flow path 58.

[0112] However, as time passes after the supply of the first gas G1C from the first adsorption tower 10C to the second adsorption tower 20 begins, the amount of N2O that can be adsorbed by the adsorbent 21 installed in the second adsorption tower 20 eventually reaches its limit, and the N2O concentration of the second gas G2 that has passed through the second adsorption tower 20 begins to show an increasing trend.

[0113] When the gas sensor 68 detects the arrival of a first timing at which the NO concentration of the second gas G2 shows an increasing trend (Yes in step #6), the process of supplying the first gas G1C from the first adsorption tower 10C to the second adsorption tower 20 is stopped (step #7). At this timing, the three-way valve 78 may be controlled to connect the pipe 48 to the decomposition tower 30.

[0114] On the other hand, during the period in which the NO concentration of the second gas G2 does not show an increasing trend (No in step #6), the NO concentration of the second gas G2 continues to be monitored while continuing to supply the first gas G1C from the first adsorption tower 10C to the second adsorption tower 20.

[0115] The arrival of the first timing can be detected by various methods, for example, by calculating a time derivative of the change in the measured value of the NO concentration of the second gas G2 within a recent predetermined time period, and detecting that the time derivative has increased by a predetermined value or more (e.g., 20% or more) compared to the immediately preceding moving average value.

[0116] As described above in the description of the first adsorption towers 10A-10C, if the gas containing CO and NO continues to be supplied to the adsorbent 11, the NO once adsorbed by the adsorbent 11 will be pushed out by the CO. However, as described above, with respect to the second adsorption tower 20, when the first timing arrives at which the NO concentration in the second gas G2 shows an increasing trend, the supply of the first gas G1C is stopped. In other words, unlike the adsorbent 11 in the first adsorption tower 10, the adsorbent 21 in the second adsorption tower 20 remains in a state in which NO is adsorbed.

[0117] This step #7 corresponds to process (e).

[0118] (Step #8) After the supply of the first gas G1C from the first adsorption tower 10C to the second adsorption tower 20 is stopped, a desorption process is performed on the adsorbent 21 in the second adsorption tower 20. Specifically, this process is performed by heating the adsorbent 21, depressurizing the adsorbent 21, or the like. According to the example of the gas treatment system 1 shown in FIG. 1 , the adsorbent 21 in the second adsorption tower 20 is heated by the heating unit 90 using the high-temperature fluid E1 flowing through the pipe 91. As a result, the substance adsorbed to the adsorbent 21 in the second adsorption tower 20 is desorbed from the adsorbent 21. At this point, the pipe 48 is connected to the decomposition tower 30, and the gas (second gas G2) discharged from the second adsorption tower 20 by desorption from the adsorbent 21 is sent to the decomposition tower 30.

[0119] In this specification, the gas discharged from the second adsorption tower 20 is referred to as the "second gas G2," but the second gas G2 discharged from the second adsorption tower 20 in step #8 has a different meaning from the second gas G2 discharged from the second adsorption tower 20 during the time period when the first gas G1C is being sent to the second adsorption tower 20 (i.e., the time period before the first timing arrives).

[0120] The latter second gas G2 is a gas obtained by passing the first gas G1C through the second adsorption tower 20, and is a gas with a low NO concentration because NO has been adsorbed by the adsorbent 21 in the second adsorption tower 20. As described above, this gas can be discharged to the outside via the discharge flow path 58.

[0121] In contrast, the former second gas G2 is a gas obtained after desorbing the N2O adsorbed by the adsorbent 21 in the second adsorption tower 20, and therefore is a gas with an extremely increased concentration of N2O compared to the gas to be treated Gt.

[0122] When performing this desorption step, if the substances adsorbed to the adsorbent 21 are exhausted in an airflow, for example, air or a buffer gas may be introduced into the second adsorption tower 20 through a gas inlet (not shown) provided in the second adsorption tower 20. When these modes are performed, the second gas G2 containing the adsorbable substances such as NO adsorbed to the adsorbent 21 is sent to the decomposition tower 30 via the pipe 48. This step #8 may be realized by applying a negative pressure (typically a vacuum state) to the second adsorption tower 20 and drawing the substances adsorbed to the adsorbent 21 to the pipe 48 side.

[0123] This step #8 corresponds to process (j).

[0124] (Step #9) As described above, in step #8, the second gas G2 discharged from the second adsorption tower 20 in a state in which the substances adsorbed on the adsorbent 21 in the second adsorption tower 20 have been desorbed is a gas containing NO at a higher concentration than the gas to be treated Gt. This second gas G2 is sent to the decomposition tower 30 via the pipe 48, and the NO contained in the second gas G2 is decomposed in the decomposition tower 30. This step #9 corresponds to the process (k).

[0125] As described above, the first gas G1C supplied from the first adsorption tower 10C to the second adsorption tower 20 has a significantly higher NO concentration than the gas Gt to be treated, and therefore the proportion of NO contained in the substance adsorbed to the adsorbent 21 in the second adsorption tower 20 is increased. Therefore, by performing the desorption process on this adsorbent 21 in step #8, the NO concentration of the second gas G2 exhausted from the second adsorption tower 20 becomes extremely high. Therefore, in step #9, the NO contained in the second gas G2 at a high concentration is decomposed in the decomposition tower 30, thereby enabling efficient decomposition of NO with a high GWP.

[0126] Fig. 4 is a cross-sectional view schematically illustrating an example of the configuration of the decomposition tower 30. In the example shown in Fig. 4, the decomposition tower 30 is equipped with a decomposition device 80 consisting of an excimer lamp. Fig. 5 corresponds to a schematic side view of the excimer lamp constituting the decomposition device 80 in this example, and Fig. 6 corresponds to a cross-sectional view taken along line A1-A1 in Fig. 5.

[0127] This excimer lamp includes a tube 81 and a pair of electrodes (82a, 82b). In this example, the tube 81 has a double-tube structure. More specifically, as shown in Fig. 6, the tube 81 has a cylindrical outer tube 81a located on the outside, and a cylindrical inner tube 81b located inside the outer tube 81a and coaxially with the outer tube 81a, with an outer diameter smaller than the inner diameter of the outer tube 81a.

[0128] 5 and 6 show an example of an excimer lamp (decomposition device 80) in which the tube axis direction of the tube body 81 is arranged along the flow direction d1 of the second gas G2. However, the tube axis direction of the tube body 81 does not necessarily need to be parallel to the flow direction d1 of the second gas G2, and these drawings are merely an example. Hereinafter, for convenience, the flow direction d1 of the second gas G2 and the tube axis direction of the tube body 81 are assumed to be parallel, and in order to avoid an increase in the number of symbols, the same symbol "d1" as the flow direction is used for the tube axis direction.

[0129] The outer tube 81a and the inner tube 81b are both sealed at their ends in the tube axis direction d1 (not shown), and a light-emitting space having a ring shape (here, a circular ring shape) when viewed from the tube axis direction d1 is formed between them. A light-emitting gas 83G that forms excimer molecules by discharge is sealed in this light-emitting space.

[0130] An electrode 82a is disposed on the outer wall of the outer tube 81a. In this embodiment, the electrode 82a has a mesh or linear shape. A rod-shaped electrode 82b extends along the axial direction of the tube body 81 and is inserted into the inner tube 81b.

[0131] The outer tube 81a and the inner tube 81b are made of a dielectric material such as synthetic quartz glass that is transparent to the ultraviolet light L1. The electrodes (82a, 82b) are made of a metal material such as stainless steel, aluminum, copper, tungsten, titanium, or nickel.

[0132] When a high-frequency AC voltage of, for example, about 1 kHz to 5 MHz is applied between the electrodes (82a, 82b) via a power supply (not shown), the voltage is applied to the light-emitting gas 83G via the tube body 81. At this time, a discharge plasma is generated in the discharge space in which the light-emitting gas 83G is sealed, and the atoms of the light-emitting gas 83G are excited to an excimer state, and excimer light emission occurs when these atoms transition to the ground state. In other words, ultraviolet light L1 derived from excimer light is generated due to the dielectric barrier discharge.

[0133] The material of the light emitting gas 83G determines the wavelength of the ultraviolet light L1 emitted from the tube 81. When a gas containing xenon (Xe) is used as the light emitting gas 83G, the excimer light emission becomes ultraviolet light L1 having a main peak wavelength in the vicinity of 172 nm.

[0134] The wavelength of the ultraviolet light L1 can be changed by changing the substance used as the luminous gas 83G. For example, ArBr (main peak wavelength near 165 nm), ArCl (main peak wavelength near 175 nm), F (main peak wavelength near 153 nm), etc. can be used as the luminous gas 83G. Here, a case where the luminous gas 83G is a gas containing Xe will be described.

[0135] As described above, in the excimer lamp of this embodiment, the electrode 82a disposed on the outer wall of the outer tube 81a has a mesh shape. Therefore, gaps exist in the electrode 82a, and the ultraviolet light L1 is extracted toward the outside of the outer tube 81a through these gaps. This ultraviolet light L1 is irradiated onto the second gas G2 flowing through the decomposition tower 30.

[0136] When the ultraviolet light L1 is irradiated onto the N2O contained in the second gas G2, the reaction of the following formula (1) occurs. 1 D) is excited atomic oxygen. The left side of equation (1) conveniently expresses that ultraviolet light of wavelength λ (corresponding to frequency ν) is absorbed by N2O. The same applies to the following equations. N2O + hν → N2 + O( 1 D) ...(1)

[0137] Theoretically, the decomposition reaction of the above formula (1) occurs when ultraviolet light L1 with a wavelength λ of 340 nm or less is used. However, since the absorption cross section of NO for light with a wavelength of 200 nm or more is small, it is preferable to use light with a wavelength of less than 200 nm, which has a relatively large absorption cross section, in order to promote the decomposition reaction of the formula (1).

[0138] As described above, when the gas Gt to be treated is generated from farm soil, livestock excrement management areas, waste management areas, waste incineration plants, wastewater treatment plants, etc., the gas Gt contains O2 derived from the air. In other words, it is assumed that the second gas G2 discharged from the second adsorption tower 20 due to the gas Gt to be treated being supplied to the first adsorption tower 10 also contains O2. The O2 contained in the second gas G2 absorbs the ultraviolet light L1. When the wavelength λ of the ultraviolet light L1 is less than 242 nm, the reaction of the following formula (2) proceeds. When the wavelength λ is less than 175 nm, the reaction of the following formula (3) proceeds in addition to the reaction of the following formula (2). In formula (2), O( 3 P) is atomic oxygen in the ground state. O2 + hν → O( 3 P) + O( 3 P) ‥‥(2) O2 + hν → O( 1 D) + O( 3 P) ... (3)

[0139] Some of the O atoms generated in the formulas (2) and (3) react with oxygen (O2) contained in the second gas G2 to generate ozone (O3) according to the following formula (4). In formula (4), M represents a third body (the same applies below). In formula (4), O( 1 D) and O( 3 O + O2 + M → O3 + M (4)

[0140] When ultraviolet light L1 is absorbed by the ozone (O3) obtained by the above formula (4), the reaction of the following formula (5) proceeds: O3 + hν → O( 1 D) + O2 (5)

[0141] Excited state atomic oxygen O( 1 Since NO) is a highly reactive substance, when it comes into contact with NO contained in the second gas G2, the reaction of formula (6) or (7) occurs. NO + O ( 1 D) → O2 + N2 ‥‥(6) N2O + O( 1 D) → NO + NO (7)

[0142] As described above, when the second gas G2 is supplied to the decomposition tower 30, NO in the second gas G2 is decomposed by the reactions of the above-described formulas (1), (6), and (7), etc. As a result, the treated gas Gd (see FIGS. 1 and 4) that has passed through the decomposition tower 30 contains substances with low GWP, and therefore can be discharged to a subsequent stage (or the atmosphere) via the pipe 49 (see FIGS. 1 and 4).

[0143] 4, the decomposition device 80, which is made up of an excimer lamp, generates a decomposition space 31 corresponding to the ultraviolet irradiation region, and the second gas G2 passes through this decomposition space 31, thereby decomposing NO in the second gas G2. For example, the operation of the decomposition device 80 may be started at the timing when the execution of step #9 is started. More specifically, if the decomposition device 80 is made up of an excimer lamp, the excimer lamp may be energized at this timing.

[0144] When the decomposition device 80 is configured with an excimer lamp, the structure of the excimer lamp illustrated in FIGS. 5 and 6 is merely an example.

[0145] For example, as shown in Fig. 7, an excimer lamp may have a single tube 81. Fig. 7 is a schematic cross-sectional view of an excimer lamp illustrated similarly to Fig. 6. The tube 81 is sealed at its end in the longitudinal direction, i.e., in the tube axis direction d1 (not shown), and a light-emitting gas 83G is sealed in the internal space. A mesh-shaped or linear electrode 82a is disposed on the outer wall surface of the tube 81, and a rod-shaped electrode 82b is disposed inside (inside) the tube 81.

[0146] As another example, as shown in Figures 8 to 9, a configuration can be employed in which both electrodes (82a, 82b) are disposed on the outer wall surface of a tube 81 of an excimer lamp. Figures 8 to 9 are drawings that schematically show the structure of an excimer lamp of this alternative configuration example, with Figure 8 corresponding to a plan view and Figure 9 corresponding to a cross-sectional view taken along line A2-A2 in Figure 8. The excimer lamp shown in Figures 8 and 9 has a single tube 81, and both electrodes (82a, 82b) are disposed on the outer wall surface of the tube 81 so as to face each other across the tube 81. The electrodes (82a, 82b) are both mesh-shaped or linear-shaped so as not to interfere with the extraction of ultraviolet light L1 generated within the tube 81 to the outside of the tube 81.

[0147] Furthermore, the decomposition device 80 for generating the decomposition space 31 as described above is not limited to an excimer lamp. As another example, the decomposition device 80 may be configured by an atmospheric pressure plasma generator, as illustrated in Figures 10 and 11 .

[0148] 10 and 11 are drawings showing a structural example of an atmospheric pressure plasma generator as a decomposition device 80, with Fig. 10 corresponding to a side view of the atmospheric pressure plasma generator and Fig. 11 corresponding to a cross-sectional view taken along line A3-A3 in Fig. 10. Unlike an excimer lamp, this atmospheric pressure plasma generator does not have a light-emitting gas 83G sealed inside the tubular body 81. Also, in Fig. 10, a second gas G2 is caused to flow inside the tubular body 81. The rest of the configuration is similar to that of the excimer lamp described above with reference to Figs. 5 and 6.

[0149] When a voltage is applied between the two electrodes (82a, 82b) from a power supply (not shown), a dielectric barrier discharge occurs within the tubular body 81. As a result, a dielectric barrier discharge occurs in the second gas G2 flowing through the decomposition space 31 formed within the tubular body 81, and the second gas G2 is converted into plasma.

[0150] When NO contained in the second gas G2 flows through the atmospheric pressure plasma space, the reaction of the following formula (8) occurs. In formula (8), AP means that energy is added by atmospheric pressure plasma. This also applies to the following formulas. In formulas (8) to (10) below, O( 1 D) and O( 3Both NO and AP are expressed as atomic oxygen O. NO + AP → N + O (8)

[0151] The oxygen (O2) contained in the second gas G2 undergoes the reaction shown in formula (9) below when flowing through the atmospheric pressure plasma space: O2 + AP → O + O (9)

[0152] A portion of the atomic oxygen O generated in formula (9) reacts with oxygen (O2) contained in the second gas G2 to generate ozone (O3) according to formula (4) above. Formula (4) is shown below again: O + O2 + M → O3 + M (4)

[0153] When energy from atmospheric pressure plasma is applied to the O3 obtained by the above formula (4), the reaction shown in formula (10) below proceeds: O3 + AP → O + O2 (10)

[0154] That is, when the second gas G2 flows through the decomposition space 31 formed by the atmospheric pressure plasma generator, O3 is introduced into the second gas G2, and O( 1 D) is generated. As a result, O( 1 When D) comes into contact with N2O contained in the second gas G2, N2O is decomposed via the reaction of formula (6) or (7).

[0155] (Step #10) After the decomposition process of the second gas G2 has progressed sufficiently, if the gas to be treated Gt remains (Yes in step #10), the process returns to step #4, and the first gas G1C from the first adsorption tower 10C is again supplied to the second adsorption tower 20, and the process is repeated. If all of the gas to be treated Gt has been treated (No in step #10), the process is completed.

[0156] [Modifications] Hereinafter, modifications of the method for treating the target gas Gt by the gas treatment system 1 of the present embodiment will be described.

[0157] <1> In step #7, when the supply of the first gas G1C from the first adsorption tower 10C to the second adsorption tower 20 is stopped, the supply of the gas to be treated Gt from the gas supply source 3 to the first adsorption tower 10A may also be stopped.

[0158] <2> As described above, in the gas treatment system 1, the adsorbent 11 in the first adsorption tower 10, which is arranged upstream of the second adsorption tower 20, predominantly adsorbs the CO contained in the gas to be treated Gt, and the second gas G2, which has a sufficiently higher NO concentration than the gas to be treated Gt, is supplied to the second adsorption tower 20. From this perspective, the first adsorption unit 18 may be configured to predominantly adsorb the CO contained in the gas to be treated Gt at a stage before the first gas G1 is supplied to the second adsorption tower 20.

[0159] 12, the first adsorption unit 18 may be configured to include a single first adsorption tower 10 that is large in size relative to the gas flow direction. In this case, the single first adsorption tower 10 can be regarded as a state in which multiple first adsorption tower 10 sections that are virtually separated relative to the gas flow direction are connected in series.

[0160] 1, the first gas G1 discharged from the first adsorption towers 10, 10, ... is sent directly to the second adsorption tower 20. As described above, when the supply of the gas to be treated Gt continues, CO2 is predominantly adsorbed onto the adsorbent 11 in the first adsorption towers 10, 10, ...

[0161] The adsorbent 11 in the first adsorption towers 10, 10, ... may be configured to desorb the adsorbed substance at a predetermined timing in order to recover the adsorption performance. In the embodiment shown in FIG. 1 , the adsorbent 11 is heated by the heating unit 90, whereby the substance adsorbed to the adsorbent 11 is desorbed and discharged as a first gas G1. This first gas G1 is a gas containing a higher concentration of CO2 than the gas to be treated Gt. During the execution of this desorption process, the supply of the gas to be treated Gt may be stopped.

[0162] In the treatment system 1 shown in Figure 1, the first gas G1 obtained by desorption from the adsorbent 11 in the first adsorption tower 10 passes through the second adsorption tower 20 and is then discharged to the outside via the discharge flow path 58.

[0163] 13 , exhaust flow paths (51, 52, 53) may be provided for the pipes (41, 42, 43) connected to the respective first adsorption towers 10. In this case, the first gas G1 obtained by desorption from the adsorbent 11 in the first adsorption tower 10 can be discharged through the exhaust flow path arranged immediately after the first adsorption tower 10.

[0164] FIG. 13 is a block diagram schematically illustrating another embodiment of the gas processing system 1. In the gas processing system 1 illustrated in FIG. 13, each of the pipes (41, 42, 43) is provided with a three-way valve (71, 72, 73). The three-way valve 71 is provided at the connection point between the pipe 41 and the exhaust flow path 51 and is configured to be able to switch between sending the gas flowing through the pipe 41 to the first adsorption tower 10B or to the exhaust flow path 51. The three-way valve 72 is provided at the connection point between the pipe 42 and the exhaust flow path 52 and is configured to be able to switch between sending the gas flowing through the pipe 42 to the first adsorption tower 10C or to the exhaust flow path 52. The three-way valve 73 is provided at the connection point between the pipe 43 and the exhaust flow path 53 and is configured to be able to switch between sending the gas flowing through the pipe 43 to the second adsorption tower 20 or to the exhaust flow path 53.

[0165] Furthermore, in the gas processing system 1 shown in Fig. 13, gas sensors (61, 62, 63) are additionally provided in the pipes (41, 42, 43), respectively, compared to the gas processing system 1 shown in Fig. 1. The details are as follows.

[0166] 13 , a gas sensor 61 is provided in a pipe 41 connecting a first adsorption tower 10A and a first adsorption tower 10B located downstream of the first adsorption tower 10A. The gas sensor 61 is configured to be able to measure the CO concentration of the first gas G1 (G1A) flowing out from the first adsorption tower 10A. The gas sensor 61 may be installed inside the first adsorption tower 10A and downstream of the adsorbent 11 in the first adsorption tower 10A.

[0167] 13 , a gas sensor 62 is provided in a pipe 42 connecting a first adsorption tower 10B and a first adsorption tower 10C located downstream of the first adsorption tower 10B. The gas sensor 62 is configured to be capable of measuring the CO concentration of the first gas G1 (G1B) flowing out from the first adsorption tower 10B. The gas sensor 62 may be installed inside the first adsorption tower 10B and downstream of the adsorbent 11 in the first adsorption tower 10B.

[0168] 13 , a gas sensor 63 is provided in a pipe 43 connecting a first adsorption tower 10C and a second adsorption tower 20 located downstream of the first adsorption tower 10C. The gas sensor 63 is configured to be capable of measuring the CO concentration of the first gas G1 (G1C) flowing out from the first adsorption tower 10C. The gas sensor 63 may be installed inside the first adsorption tower 10C and downstream of the adsorbent 11 in the first adsorption tower 10C.

[0169] When desorbing a substance adsorbed on the adsorbent 11 in the first adsorption tower 10, the three-way valves (71, 72, 73) are controlled to connect the pipes (41, 42, 43) to the discharge flow paths (51, 52, 53). This allows the first gas G1 obtained by desorption to be discharged via the discharge flow paths (51, 52, 53). More specifically, during desorption from the adsorbent 11 of the first adsorption tower 10A, the first gas G1 having a relatively high CO2 concentration discharged from the first adsorption tower 10A is discharged from the discharge flow path 51. During desorption from the adsorbent 11 of the first adsorption tower 10B, the first gas G1 having a relatively high CO2 concentration discharged from the first adsorption tower 10B is discharged from the discharge flow path 52. During desorption from the adsorbent 11 of the first adsorption tower 10C, the first gas G1 having a relatively high CO2 concentration discharged from the first adsorption tower 10C is discharged from the discharge flow path 53.

[0170] Each gas sensor (61, 62, 63) monitors the CO concentration of the first gas G1 discharged from each first adsorption tower (10A, 10B, 10C) during desorption. As the desorption process is performed, the CO concentration of the first gas G1 gradually decreases. When it is confirmed that the measured CO concentration of the first gas G1 has fallen below a predetermined threshold, it can be determined that the desorption process of the adsorbent 11 has been completed, and the three-way valves (71, 72, 73) are controlled to connect the pipes (41, 42, 43) to the subsequent adsorption towers (10B, 10C, 20). Thereafter, the process returns to step #2, and the gas to be treated Gt is again supplied to the first adsorption tower 10A.

[0171] In the above description, the gas sensors (61, 62, 63) are assumed to be capable of measuring the CO concentration of the flowing gas, but they may also be capable of measuring the NO concentration. In this case, since it is possible to monitor the NO concentration of the first gas G1 (G1A, G1B, G1C) discharged from each of the first adsorption towers 10 (10A, 10B, 10C), it is also possible to check whether each of the first adsorption towers 10 (10A, 10B, 10C) is functioning properly while the treatment target gas Gt is being treated.

[0172] It is also possible to adsorb CO2 by providing a general-purpose CO2 adsorbent (not shown) downstream of the exhaust flow paths (51, 52, 53). Similarly, a general-purpose CO2 adsorbent (not shown) may be provided downstream of the exhaust flow path 58.

[0173] 13 illustrates an embodiment in which the discharge flow paths (51, 52, 53) are provided corresponding to all of the first adsorption towers 10, but the discharge flow path may be connected only to the most downstream first adsorption tower 10 (10C). In this case, it is optional whether or not to provide the gas sensors 61 and 62.

[0174] <4> As shown in Figures 14 and 15 , first adsorption units 18 including first adsorption towers 10, 10, ... connected in series may be provided in parallel in multiple stages, and the first gas G1 may be sent from each of the first adsorption units 18 to the same second adsorption tower 20. Note that although elements within the first adsorption unit 18 are not shown in Figures 14 and 15 , for example, elements similar to those in the first adsorption unit 18 shown in Figure 13 may be arranged therein.

[0175] Hereinafter, when it is necessary to distinguish between the first suction units 18, 18, . . . , they will be referred to as "first suction unit 18A" and "first suction unit 18B."

[0176] For example, in the gas processing system 1 shown in FIG. 14, it is possible to switch the first adsorption unit 18 to which the gas supply source 3 supplies the gas Gt to be processed by controlling the on-off valves (35, 36).

[0177] As described above, it is considered that the adsorbents 11 in the first adsorption towers 10, 10, ... constituting the first adsorption unit 18 have reached their adsorption limits at the time when the second gas G2 is decomposed in the decomposition tower 30. Even if the gas to be treated Gt continues to be supplied to the adsorbent 11 in the first adsorption tower 10 in this state, it is difficult for the adsorbent 11 to additionally adsorb the CO2 contained in the gas to be treated Gt.

[0178] Therefore, the desorption process of the adsorbed substance may be performed at a predetermined timing for each of the adsorbents 11 in the first adsorption towers 10, 10, . . . that constitute the first adsorption unit 18.

[0179] For example, during a time period in which the on-off valve 35 is closed and the supply of the gas to be treated Gt to the first adsorption unit 18A is stopped, a desorption process is performed on each of the adsorbents 11 in the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18A. When the desorption process is performed on each of the adsorbents 11 in the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18A, the on-off valve 37 may be closed, and the gas obtained by desorption may be discharged to the outside of the system without being sent to the second adsorption tower 20.

[0180] While the desorption process is being performed on each adsorbent 11 in the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18A, the on-off valve 36 and the on-off valve 38 are kept open. As a result, the gas to be treated Gt is supplied to the first adsorption unit 18B, and the processes of steps #1 to #10 described above are performed on the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18B. Because the on-off valve 38 is open, the first gas G1 discharged from the first adsorption tower 10 located furthest downstream in the first adsorption unit 18B is sent to the second adsorption tower 20 via the pipe 43.

[0181] After desorption from each of the adsorbents 11 in the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18A is completed, the on-off valves 35 and 37 are opened, and the treatment target gas Gt is supplied to the first adsorption unit 18A, while the above-mentioned processes of steps #1 to #10 are performed on the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18A. Furthermore, during this time period, the on-off valves 36 and 38 are closed, thereby stopping the supply of the treatment target gas Gt to the first adsorption unit 18B, and desorption from each of the adsorbents 11 in the first adsorption towers 10, 10, ... that constitute the first adsorption unit 18B can be performed.

[0182] In other words, according to the gas processing system 1 shown in Figure 14, the desorption process of the adsorbent 11 in the first adsorption towers 10, 10, ... and the processing of the gas Gt to be processed using the first adsorption towers 10, 10, ... can be carried out in parallel.

[0183] In the gas processing system 1 shown in FIG. 15, a control mode similar to that of the gas processing system 1 shown in FIG. 14 can be realized by controlling an on-off valve (not shown) provided in each gas supply source 3 to control whether or not the gas to be processed Gt from the gas supply source 3 is supplied to the piping 40.

[0184] 14, the gas to be treated Gt may be supplied simultaneously to both the first adsorption unit 18A and the first adsorption unit 18B. The same applies to FIG.

[0185] <5> In the gas processing system 1 shown in Fig. 14 , first adsorption units 18 including first adsorption towers 10, 10, ... connected in series are provided in parallel in multiple stages, and the first gas G1 is sent from each of the first adsorption units 18 to the same second adsorption tower 20. In contrast, as in the gas processing system 1 shown in Fig. 16 , the second adsorption tower 20 and the decomposition tower 30 may also be provided in parallel in multiple stages.

[0186] According to this configuration, for example, while the desorption step (step #8) is being performed for the second adsorption tower 20 connected to the first adsorption unit 18A, steps #4 to #6 can be performed in parallel using the second adsorption tower 20 connected to another first adsorption unit 18B. In other words, this enables continuous treatment of the treatment target gas Gt without stopping the supply of the treatment target gas Gt. The gas treatment system 1 shown in Figure 16 is based on the gas treatment system 1 shown in Figure 14, but can naturally also be applied to the gas treatment system 1 shown in Figure 15.

[0187] <6> In the gas treatment system 1 shown in Fig. 1 , similarly to the gas treatment system 1 shown in Fig. 13 , gas sensors (61, 62, 63) may be additionally provided in the pipes (41, 42, 43), respectively. By making it possible to measure the NO concentration of the flowing gas using the gas sensors (61, 62, 63), it is possible to monitor the NO concentration of the first gas G1 (G1A, G1B, G1C) discharged from each of the first adsorption towers 10 (10A, 10B, 10C). This makes it possible to check, for example, whether each of the first adsorption towers 10 (10A, 10B, 10C) is functioning properly while the treatment target gas Gt is being treated.

[0188] In the above-described aspect of the present embodiment, the gas sensors (60, 61, 62, 63, 68) may be configured to be capable of measuring at least both the N2O concentration and the CO2 concentration of the gas flowing through the gas. The same applies to the second embodiment described below.

[0189] Second Embodiment A second embodiment of the gas processing method according to the present invention will be described, focusing mainly on the differences from the first embodiment.

[0190] 17 is a block diagram schematically illustrating an example of a gas processing system 1 capable of executing the gas processing method of the present embodiment. The gas processing system 1 illustrated in FIG. 17 differs from the gas processing system 1 illustrated in FIG. 13 in that it includes bypass flow paths 101, 102, and 103.

[0191] More specifically, it is as follows.

[0192] The bypass flow path 101 is configured to connect the pipe 40 and the pipe 41 so as to bypass the first adsorption tower 10A, thereby enabling connection between the upstream side of the first adsorption tower 10A (here, the gas supply source 3) and the next-stage adsorption tower (here, the first adsorption tower 10B) located immediately after the first adsorption tower 10A without going through the first adsorption tower 10A.

[0193] The bypass flow path 102 is configured to connect the pipes 41 and 42 so as to bypass the first adsorption tower 10B, thereby enabling connection between the upstream side of the first adsorption tower 10B (here, the first adsorption tower 10A) and the next-stage adsorption tower (here, the first adsorption tower 10C) located immediately after the first adsorption tower 10B without going through the first adsorption tower 10B.

[0194] The bypass flow path 103 is configured to connect the pipe 42 and the pipe 43 so as to bypass the first adsorption tower 10C, thereby enabling connection between the upstream side of the first adsorption tower 10C (here, the first adsorption tower 10B) and the next-stage adsorption tower (here, the second adsorption tower 20) located immediately after the first adsorption tower 10C without going through the first adsorption tower 10C.

[0195] A three-way valve 74 is provided in the pipe 40, and by adjusting the aperture of the three-way valve 74, it is possible to control whether the gas flowing through the pipe 40 is directed toward the first adsorption tower 10A or the bypass flow path 101. A three-way valve 75 is provided in the pipe 41, and by adjusting the aperture of the three-way valve 75, it is possible to control whether the gas flowing through the pipe 41 is directed toward the first adsorption tower 10B or the bypass flow path 102. A three-way valve 76 is provided in the pipe 42, and by adjusting the aperture of the three-way valve 76, it is possible to control whether the gas flowing through the pipe 42 is directed toward the first adsorption tower 10C or the bypass flow path 103.

[0196] Although FIG. 17 illustrates a case where three-way valves (74, 75, 76) are provided, the present invention is not limited to the three-way valves (74, 75, 76) as long as it is possible to control whether the gas flowing from the upstream side is to be passed toward the immediately succeeding first adsorption towers 10, 10, ... or toward the bypass flow paths 101, 102, ... that bypass the immediately succeeding first adsorption towers 10, 10, ....

[0197] 18 is a flowchart schematically illustrating the procedure for executing the gas processing method according to this embodiment. In the following description, the step numbers shown in the flowchart in FIG. 18 will be referred to as appropriate. In the flowchart in FIG. 18, the same steps as those in the flowchart in FIG. 2 are assigned the same step numbers.

[0198] The gas processing method of this embodiment differs from the first embodiment in that steps #11 to #18 are additionally performed.

[0199] (Step #11) As described above in the first embodiment, in step #3, the first gas G1 is sent from the first adsorption tower 10A to the first adsorption tower 10B. In the gas processing system 1 shown in Fig. 17, similarly to the gas processing systems 1 shown in Figs. 1 and 13, the first adsorption unit 18 includes three first adsorption towers 10A, 10B, and 10C, and therefore the first gas G1 is also sent from the first adsorption tower 10B to the first adsorption tower 10C.

[0200] In step #11, the NO concentration is measured in the first gas G1 discharged from each of the first adsorption towers 10. As a more detailed example, the gas sensor 61 monitors the NO concentration in the first gas G1A discharged from the first adsorption tower 10A, the gas sensor 62 monitors the NO concentration in the first gas G1B discharged from the first adsorption tower 10B, and the gas sensor 63 monitors the NO concentration in the first gas G1C discharged from the first adsorption tower 10C.

[0201] This step #11 corresponds to process (f).

[0202] (Steps #12, #13, and #14) In step #12, the arrival of a second timing is detected, at which the NO concentration in the first gas G1 discharged from each first adsorption tower 10 satisfies a predetermined condition. This second timing corresponds to the timing at which it can be determined that the adsorbent 11 contained in each first adsorption tower 10 no longer functions to adsorb additional NO.

[0203] As described above in the first embodiment, the adsorbent 11 in the first adsorption tower 10 initially adsorbs CO and NO contained in the supplied gas (the target gas Gt or the first gas G1). However, as the supply of the target gas continues, the adsorbed NO is pushed out by CO. After this, the adsorbent 11 in the first adsorption tower 10 can no longer adsorb additional NO. In other words, with respect to the adsorbent 11 in the first adsorption tower 10A, the NO concentration in the first gas G1A discharged from the first adsorption tower 10A becomes substantially equal to the NO concentration in the target gas Gt supplied to the first adsorption tower 10A.

[0204] For example, by comparing the NO concentration of the gas Gt to be treated with the NO concentration of the first gas G1A, if the difference between the two is less than a predetermined value (for example, less than 10%) of the NO concentration of the gas Gt to be treated, it can be determined that the second timing has arrived for the adsorbent 11 in the first adsorption tower 10A.

[0205] The same discussion can be applied to the adsorbent 11 in the first adsorption tower 10B and the adsorbent in the first adsorption tower 10C. That is, when the difference value between the NO concentration of the first gas G1A and the NO concentration of the first gas G1B is equal to or less than a predetermined value relative to the NO concentration of the first gas G1A, it may be determined that the second timing has arrived for the adsorbent 11 in the first adsorption tower 10B. Similarly, when the difference value between the NO concentration of the first gas G1B and the NO concentration of the first gas G1C is equal to or less than a predetermined value relative to the NO concentration of the first gas G1B, it may be determined that the second timing has arrived for the adsorbent 11 in the first adsorption tower 10C.

[0206] If it is confirmed in step #12 that the second timing has arrived (Yes in step #12), the first adsorption tower 10 containing the corresponding adsorbent 11 is bypassed (step #13). Note that, as long as it is not confirmed in step #12 that the second timing has arrived, steps #3 and #11 are continuously executed.

[0207] Here, an example will be described in which the first adsorption tower 10 containing the adsorbent 11 for which the arrival of the second timing has been confirmed is the first adsorption tower 10A. In this case, the first adsorption tower 10A corresponds to the "first adsorption tower to be regenerated."

[0208] Specifically, the pipe 40 upstream of the first adsorption tower 10A corresponding to the first adsorption tower to be regenerated is connected to a bypass flow path 101 that bypasses the first adsorption tower 10A. Then, without delay, the connection between the pipe 40 and the first adsorption tower 10A is interrupted (step #14). As a result, the gas supplied to the first adsorption tower to be regenerated (here, the gas to be treated Gt supplied to the first adsorption tower 10A) is not supplied to the first adsorption tower 10A, but is instead supplied to the subsequent first adsorption tower 10B via the bypass flow path 101.

[0209] These steps #13 and #14 correspond to process (g).

[0210] (Step #15) After step #14 is completed, the first adsorption tower to be regenerated (first adsorption tower 10A in this case) is completely isolated from the gas flow. In this state, a desorption process of the substances adsorbed by the adsorbent 11 contained in the first adsorption tower to be regenerated is performed.

[0211] Specifically, this is performed by a method of heating the adsorbent 11 in the first adsorption tower 10A, a method of depressurizing the adsorbent 11, or the like. According to the example of the gas processing system 1 shown in Fig. 17 , the adsorbent 11 in the first adsorption tower 10A is heated by the heating unit 90 using a high-temperature fluid E1 flowing through a pipe 91. As a result, the substance adsorbed to the adsorbent 11 in the first adsorption tower 10A is desorbed from the adsorbent 11.

[0212] When performing this desorption step, if the substance adsorbed to the adsorbent 11 is exhausted in an airflow, air or a buffer gas may be introduced into the first adsorption tower 10A through a gas inlet (not shown) provided in the first adsorption tower 10A. In this case, the three-way valve 71 may be controlled to connect the exhaust outlet of the first adsorption tower 10A to the exhaust flow path 51. In this case, a first gas G1A containing the adsorbed substance adsorbed to the adsorbent 11 in the first adsorption tower 10A is exhausted to the outside via the exhaust flow path 51. This first gas G1A contains almost no NO and has a higher CO concentration than the target gas Gt.

[0213] In this specification, the gas discharged from the first adsorption tower 10 is referred to as the "first gas G1." However, the first gas G1 discharged from the first adsorption tower to be regenerated in step #15 has a different meaning from the first gas G1 discharged from the first adsorption tower 10 toward the subsequent first adsorption tower 10 or second adsorption tower 20 during the execution of steps #3 to #4.

[0214] The latter first gas G1 is a gas obtained by passing the supplied treatment target gas Gt or first gas G1 (hereinafter collectively referred to as the "supply gas") through the first adsorption tower 10. In other words, when the adsorbent 11 in the first adsorption tower 10 exhibits high adsorption performance, the first gas G1 referred to here is a gas obtained after CO and NO contained in the supply gas are adsorbed, and is a gas with reduced CO and NO concentrations compared to the supply gas. Furthermore, when the adsorbent 11 in the first adsorption tower 10 has reached or is close to its adsorption limit, the first gas G1 referred to here is a gas with CO and NO concentrations approximately equivalent to those of the supply gas.

[0215] In contrast, the former first gas G1 is a gas obtained after desorbing the CO2 adsorbed by the adsorbent 11 in the first adsorption tower 10 (the first adsorption tower to be regenerated), and is therefore expected to have a higher CO2 concentration than the gas Gt to be treated.

[0216] As described above, at the time of step #15, CO is predominantly adsorbed to the adsorbent 11 in the first adsorption tower 10A, and almost no NO is adsorbed. Therefore, by performing step #15, the first gas G1 containing CO and substantially no NO is discharged from the first adsorption tower 10A to the outside via the discharge flow path 51. In this case, CO can also be adsorbed by providing a general-purpose CO adsorbent (not shown) downstream of the discharge flow path 51.

[0217] In step #15, while the desorption step is being performed for the first adsorption tower 10A, the gas to be treated Gt flows into the first adsorption tower 10B through the bypass flow path 101. That is, the adsorption towers subsequent to the first adsorption tower 10A (the first adsorption tower 10B, the first adsorption tower 10C, and the second adsorption tower 20) can continue to operate. This makes it possible to regenerate the adsorbent 11 in the first adsorption tower 10A without stopping gas treatment.

[0218] This step #15 corresponds to process (h).

[0219] (Step #16, Step #17, Step #18)

[0220] The desorption process continues until a third timing arrives at which desorption from the adsorbent 11 in the first adsorption tower 10A, the first adsorption tower to be regenerated, is substantially complete (No in step #16). On the other hand, when the third timing arrives (Yes in step #16), the pipe 40 and the first adsorption tower 10A are reconnected, and the supply of the target gas Gt to the first adsorption tower 10A is resumed (step #17). Thereafter, the connection between the pipe 40 and the bypass flow path 101 is immediately interrupted (step #18).

[0221] Various methods can be used to detect the arrival of the third timing.

[0222] As an example, it can be determined that the third timing has arrived when the rate of decrease in the CO concentration of the first gas G1 discharged from the first adsorption tower 10A during the desorption process reaches or falls below a predetermined value. As described above, CO is predominantly adsorbed to the adsorbent 11 in the first adsorption tower 10A prior to the desorption process. Therefore, when the desorption process is performed, the CO adsorbed on the adsorbent 11 is desorbed and contained in the first gas G1. As the desorption process is continuously performed, the desorption rate of CO adsorbed on the adsorbent 11 decreases, and the CO concentration in the first gas G1 also tends to decrease.

[0223] As another example, the first adsorption tower 10A is provided with a weight sensor (not shown) capable of measuring the weight of the adsorbent 11, and it can be determined that the third timing has arrived when the relative value of the weight of the adsorbent 11 measured by this weight sensor with respect to a reference value reaches or falls below a predetermined value. The reference value here may be set appropriately based on the initial weight of the adsorbent 11 in the first adsorption tower 10A.

[0224] Steps #17 and #18 correspond to process (i). After step #18 is completed, the process returns to step #3.

[0225] In the above description, the case where the first adsorption tower to be regenerated is the first adsorption tower 10A has been given as an example, but it will be understood that the same description can also be applied when the first adsorption tower to be regenerated is the first adsorption tower 10B or the first adsorption tower 10C. Furthermore, it will be understood that the same processing can be performed while sequentially switching the first adsorption tower to be regenerated.

[0226] That is, it is possible to switch between steps #11 to #18, with first adsorption tower 10A as the first adsorption tower to be regenerated, steps #11 to #18, with first adsorption tower 10B as the first adsorption tower to be regenerated, and steps #11 to #18, with first adsorption tower 10C as the first adsorption tower to be regenerated, as appropriate. This allows the regeneration treatment of the adsorbent 11 in each of the first adsorption towers 10, 10, ... to be performed without stopping the treatment of increasing the NO concentration in the gas Gt to be treated.

[0227] [Modifications] In this embodiment as well, it is possible to apply the modifications described above with reference to FIGS. 14 and 15 in the first embodiment.

[0228] [Additional Remarks] The present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to provide a better understanding of the present invention, and the present invention is not necessarily limited to those having all of the configurations described. The scope of the present invention is defined by the claims, and it is intended to include all modifications within the meaning and scope of the claims.

[0229] 1: Gas treatment system 2: Adsorption system 3: Gas supply source 10, 10A, 10B, 10C: First adsorption tower 11: Adsorbent in first adsorption tower 18, 18A, 18B: First adsorption unit 20: Second adsorption tower 21: Adsorbent in second adsorption tower 30: Decomposition tower 31: Decomposition space 35, 36, 37, 38: On-off valve 40, 41, 42, 43, 48, 49: Piping 51, 52, 53, 58: Discharge flow path 60, 61, 62, 63, 68: Gas sensor 71, 72, 73, 74, 75, 76, 78: Three-way valve 80: Decomposition device 81: Tube body 81a: Outer tube 81b: Inner tube 82a, 82b: Electrode 83G : Light emitting gas 90: Heating unit 91: Pipe 101, 102, 103: Bypass flow path E1: Fluid G1, G1A, G1B, G1C: First gas G2: Second gas Gd: Processed gas Gt: Gas to be processed L1: Ultraviolet light d1: Flow direction

Claims

1. A gas treatment method comprising the steps of: (a) preparing an adsorption system including a first adsorption tower containing an adsorbent capable of adsorbing CO2 and N2O; and a second adsorption tower connected downstream of the first adsorption tower and containing an adsorbent capable of adsorbing CO2 and N2O; (b) passing a gas to be treated containing CO2 and N2O through the first adsorption tower; (c) continuously feeding the first gas that has passed through the adsorbent in the first adsorption tower into the second adsorption tower; (d) measuring the N2O concentration of a second gas located downstream of the adsorbent in the second adsorption tower in the flow direction; and (e) terminating the execution of step (c) at a predetermined first timing when the N2O concentration of the second gas measured in step (d) shows an increasing trend, wherein the first timing is when the adsorbent contained in at least the first adsorption tower located furthest upstream reaches its adsorption limit.

2. The gas treatment method according to claim 1, characterized in that the adsorption system comprises a plurality of the first adsorption towers connected in series, and step (c) is a step of feeding the first gas that has passed through the plurality of first adsorption towers into the second adsorption tower.

3. The gas treatment method according to claim 2, characterized in that the adsorption system comprises: a bypass flow path that bypasses each of the multiple first adsorption towers and is connectable to a next-stage adsorption tower, which is the first adsorption tower or the second adsorption tower, located immediately behind the multiple first adsorption towers; and an exhaust flow path that is connectable to each of the multiple first adsorption towers while flow to the next-stage adsorption tower is blocked; and the method comprises: a step (f) of measuring the N2O concentration of the first gas corresponding to each of the multiple first adsorption towers; a step (g) of connecting the upstream of a first adsorption tower to be regenerated, which is a first adsorption tower that has been confirmed to have an N2O concentration of the first gas measured in step (f) substantially identical to the concentration of the gas to be treated, with the bypass flow path corresponding to the first adsorption tower to be regenerated, thereby stopping the inflow of gas into the first adsorption tower to be regenerated; and a step (h) of desorbing the adsorbed substance adsorbed to the adsorbent in the first adsorption tower to be regenerated while connecting the first adsorption tower to the exhaust flow path corresponding to the first adsorption tower to be regenerated.

4. The gas processing method described in claim 3, characterized in that after completion of step (h), the method includes step (i) of blocking the bypass flow path corresponding to the first adsorption tower to be regenerated and connecting the upstream of the first adsorption tower to be regenerated with the first adsorption tower to be regenerated, and step (g), step (h), and step (i) are performed by sequentially switching the first adsorption tower to be regenerated from among the multiple first adsorption towers.

5. The gas treatment method according to claim 3, wherein step (h) includes a step of heating the adsorbent in the first adsorption tower to be regenerated.

6. A gas treatment method as described in claim 5, characterized in that step (h) is a step of heating the adsorbent in the first adsorption tower to be regenerated using exhaust heat supplied from outside the adsorption system.

7. The gas treatment method according to claim 3, wherein step (h) includes a step of depressurizing the adsorbent in the first adsorption tower to be regenerated.

8. The gas treatment method according to claim 1, further comprising, after step (e), a step (j) of desorbing the adsorbed substance adsorbed on the adsorbent in the second adsorption tower.

9. The gas treatment method according to claim 8, wherein step (j) includes a step of heating the adsorbent in the second adsorption tower.

10. A gas treatment method as described in claim 8, characterized in that step (j) is a step of heating the adsorbent in the second adsorption tower using exhaust heat supplied from outside the adsorption system.

11. The gas treatment method according to claim 8, wherein step (j) includes a step of depressurizing the adsorbent in the second adsorption tower.

12. The gas treatment method according to claim 8, further comprising a step (k) of decomposing N2O contained in the second gas obtained after the start of the execution of the step (j).

13. The gas treatment method according to claim 12, wherein step (k) includes a step of feeding the second gas obtained after the start of step (j) into a decomposition space formed by at least one of an ultraviolet ray irradiation region and a plasma generation region.

14. A gas treatment method according to any one of claims 1 to 13, characterized in that the adsorbent contained in the first adsorption tower and the adsorbent contained in the second adsorption tower are mainly composed of zeolite.

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